1. Field of the Invention
This invention relates generally to peristaltic pump means for pumping dialysate solution through a dialysate solution flow circuit in a hemodialysis system.
2. Description of the Prior Art
Hemodialysis systems have been in general use for a number of years in the treatment of renal disease and disability, and have proven highly effective in providing artificial kidney functions for persons whose own natural kidneys are functionally imparied. In operation of the hemodialysis system, blood containing waste substances such as for example urea, creatinine, excess electrolytic salts and water, is withdrawn from the body and flowed through a dialyzer in indirect mass transfer relationship with an aqueous dialysate solution. The dialyzer may be of various conventional types including a mass transfer member such as an extended surface elastomeric membrane or a hollow fiber bundle across which the waste substances are transferred by concentration gradient (solute impurities) or osmotic pressure (water) from the blood to the dialysate solution. From the dialyzer the blood which has thus been depleted in impurities is returned to the patient's body. The impurity-enriched dialysate solution from the dialyzer is either disposed of to waste or else is recirculated in a closed loop from a dialysate solution source through the dialyzer for renewed mass transfer from the blood to the dialysate solution. The dialysate solution employed in conventional hemodialysis treatment is an aqueous solution of various selected slats which provide the desired electrolytic balance with the blood in the dialyzer and thus prevent the loss of valuable blood constituents to the dialysate solution by diffusion and osmosis in the dialyzer.
In the general practice of dialysis, it is frequently desirable to operate the dialysis system such that the pressure in the dialysate solution on one side of the mass transfer member of the dialyzer, e.g., dialyzer membrane, is at a somewhat reduced (negative pressure) level relative to the pressure of the blood on the other side of the dialyzer mass transfer member. Such mode of operation provides a pressure differential across the mass transfer member of the dialyzer and is employed to facilitate the transport of excess water from the patient's circulatory system to the dialysate solution flow stream by osmotic pressure flow. This removal of excess water, commonly referred to as ultrafiltration, usefully provides a means for removal of water from the patient's body, such as would ordinarily be carried out with the aid of functional natural kidneys.
It is taught by the prior art to employ in the hemodialysis system a dialysate solution flow circuit including a flexible resilient tubing pumping section through which dialysate solution is pumped by peristaltic pump means having a pluraity of circumferentially spaced-apart rollers mounted on a rotable base member. In such system, the flexible resilient tubing pumping section is tensionally extended around the rollers so as to be simultaneously engaged and compressed by at least two of the rollers, with the rollers being mounted for longitudinal movement of the points of compression along the tubing during rotation of said pump base member to advance dialysate solution through the tubing.
A recently developed dialysate solution peristaltic pump of the above-described type is disclosed and claimed in U.S. Ser. No. 720,672, now U.S. Pat. No. 4,083,777, filed Sept. 7, 1976 in the name of J. T. Hutchisson. This pump is highly efficient and extremely small in size and weight. For such reasons, this prior art pump has been employed and is particularly suitable for use in portable hemodialysis systems. More specifically, the pump disclosed in Ser. No. 720,672 comprises three rollers mounted on a base member, each roller having a diameter of between 0.25 and 0.75 inch and circumferentially spaced apart at an angle of 120.degree. from the other rollers with a radial distance between the roller axis and pump head assembly base member fixed axis of from 0.50 to 1.25 inches. This prior art pump employs a flexible resilient tubing pumping section having a length, as measured longitudinally along the tubing between the anchored end segments thereof, of from 6.0 to 6.75 inches, a wall thickness of from 0.03 to 0.10 inch and an internal diameter of from 0.18 to 0.25 inch, with drive means connected to the pump head assembly for rotation thereof at a speed in the range of from 200 to 600 rpm.
Despite the advantages of high efficiency and small size and weight of the dialysate solution pump disclosed in U.S. Ser. No. 720,672, it has been found that the output flow rate of dialysate solution provided by this prior art pump varies drastically with changes in negative pressure in the dialysate solution. Such flow rate-negative pressure characteristic represents a severe operating disadvantage for the prior art pump when employed to pump dialysate solution in the hemodialysis system, for the reason that it is generally necessary to vary the level of negative pressure of the dialysate solution being pumped through the dialyzer in such system from patient to patient and, for a given patient, from dialysis to dialysis, depending on the level of ultrafiltration water removal required in a particular dialysis treatment. For example, the effect of increasing the dialysate solution negative pressure for dialysis is to correspondingly increase the pressure gradient driving force for mass transfer of water from the blood through the dialyzer mass transfer surface to the dialysate solution passed in indirect mass transfer dialyzing relationship with the blood, so as to insure complete removal of water from the blood in the course of dialysis treatment. The above-described prior art pump of U.S. Ser. No. 720,672 responds to increased negative pressure with drastically reduced output flow of dialysate solution from the pump. The resulting reduction in dialysate solution flow rate in the dialysate solution flow circuit of the hemodialysis system adversely affects the overall efficiency of the dialysis treatment and requires significantly increased treatment time to effect the required removal of inpurities from the blood being dialyzed.
Faced with the problem of widely varying dialysate solution flow rate from a single peristaltic pump with changes in dialysate solution negative pressure, the prior art has proposed various arrangements in which two peristaltic pumps are disposed in series in the dialysate solution flow circuit to maintain relatively constant flow therein despite changes in dialysate solution negative pressure. In one such arrangement, a first peristaltic pump is disposed upstream of the dialyzer, with a flow restriction device interposed between the first peristaltic pump and the dialyzer to create negative pressure in the dialysate solution flowing therethrough. A second peristaltic pump is disposed downstream of the dialyzer. The function of the second pump is to accommodate the variation in dialysate solution flow rate from the first pump with changes in negative pressure, i.e., to "smooth" the output flow rate from the first pump, so as to maintain a substantially constant flow rate in the dialysate solution flow circuit despite such negative pressure changes. Although the dual peristaltic pump arrangement is effective in overcoming the problem of variance in dialysate solution flow rate with changes in dialysate solution negative pressure, such arrangement involves the added expense and complexity of a second peristaltic pump as compared to hemodialysis systems wherein only single peristaltic pump means are employed.
Accordingly, it is an object of this invention to provide improved peristaltic pump means for pumping dialysate solution through a dialysate solution flow circuit in a hemodialysis system.
It is another object of the present invention to provide a single peristaltic pump means capable of delivering a substantially constant output flow rate of dialysate solution despite changes in the negative pressure of the dialysate solution being pumped.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.